1. Field of the Invention
The present invention relates to near-field light exposure masks, methods for manufacturing near-field light exposure masks, exposure apparatuses and methods using the near-field light exposure masks, and methods for manufacturing devices. In particular, the present invention relates to a near-field light exposure mask used for exposing a workpiece (the workpiece including a single crystal substrate such as a semiconductor wafer, a glass substrate for a liquid crystal display (LCD), or the like), a method for manufacturing a near-field light exposure mask, an exposure apparatus and method using the near-field light exposure mask mentioned above, and a method for manufacturing a device.
2. Related Background Art
In recent years, due to the trend toward miniaturization of electronic apparatuses and decrease in thickness thereof, techniques for forming high-density semiconductor devices to be mounted on electronic apparatuses have been increasingly demanded. For example, concerning the design rule for a pattern of a mask or a reticle, the size of line and space (L & S) used in mass production processes has been decreased close to 130 nm and, in addition, has been expected to be continuously decreased. The L & S is an image projected on a wafer in exposure, in which the image has the line width equal to the space width, and indicates the resolution of the exposure.
A projection exposure apparatus, which has been predominantly used in recent years, comprises (a) an illumination optical system illuminating a mask by using light flux emitted from a light source, and (b) a projection optical system provided between the mask and a workpiece which is to be exposed. In the illumination optical system, in order to obtain an area having uniform illuminance distribution, light flux emitted from the light source is introduced into a light integrator such as a fly-eye lens composed of a plurality of rod lenses, and the mask surface is illuminated in Koehler illumination by a condenser lens using a surface of the light integrator as a secondary light-emitting surface.
Resolution R of a projection exposure apparatus is represented by the following equation using a wavelength λ of a light source and the numerical aperture (NA) of the exposure apparatus:                     R        =                              k            1                    ×                      λ            NA                                              [                  Equatio          ⁢                                          ⁢          n          ⁢                                          ⁢          1                ]            where K1 is a constant.
Accordingly, when the wavelength is decreased, and when the NA is increased, the resolution is improved.
In addition, the range of focus, which can maintain imaging characteristics at a certain level, is referred to as a depth of focus (DOF), and the depth of focus DOF can be represented by the following equation:                     DOF        =                              k            2                    ×                      λ                          NA              2                                                          [                  Equatio          ⁢                                          ⁢          n          ⁢                                          ⁢          2                ]            where K2 is a constant.
Accordingly, when the wavelength is decreased, and when the NA is increased, the depth of focus becomes smaller. When the depth of focus becomes smaller, focusing becomes difficult, and as a result, improvements in substrate flatness and focus accuracy are required. Hence, a larger depth of focus is preferable in general.
It is understood from Equations 1 and 2 that a decrease in wavelength is more effective than an increase in NA. Hence, as a recent light source, conventional ultra high pressure mercury lamps have been replaced by a KrF excimer laser (wavelength of approximately 248 nm) or an ArF excimer laser (wavelength of approximately 193 nm), each of which has a wavelength shorter than that of the conventional mercury lamps described above.
However, the values of the constants k1 and k2 are generally from approximately 0.5 to 0.7, and even when a method for improving the resolution, such as a phase shift method, is used, since these constants may only become approximately 0.4, it has been difficult to improve the resolution by decreasing the constants.
In addition, it has been generally believed that the wavelength of a light source is the limitation on resolution of a projection exposure apparatus, and even when an excimer laser is used, it has been difficult for a projection exposure apparatus to form a pattern having a dimension of 0.10 μm or less. In addition, even if a light source having a shorter wavelength is present, an optical material (that is, a glass material for a lens) may not be able to transmit exposure light having the shorter wavelength described above, and as a result, a problem may arise in that exposure cannot be performed (that is, projection cannot be performed for a workpiece). That is, the transmittances of substantially all glass materials are close to zero in the far ultraviolet region.
In addition, although synthetic quartz produced by a specific production method may be applied to exposure which uses exposure light having a wavelength of approximately 248 nm, when the wavelength becomes 193 nm or less, the transmittance of the synthetic quartz is rapidly decreased. Hence, it has been very difficult to develop a practical glass material having a sufficiently high transmittance for exposure light having a wavelength of 150 nm or less, which can be used for forming a fine pattern having a dimension of 0.10 μm or less. Furthermore, since a glass material used in the far ultraviolet region is expected to satisfy certain levels of various properties, such as durability, refractive index, uniformity, optical stress, and workability, in addition to the transmittance described above, the development of a practical glass material has not been easily carried out.
In order to solve the problems described above, an exposure apparatus has been recently proposed which uses the principle of a scanning near-field microscope (SNOM) as means for performing microfabrication in which a dimension of 0.1 μm or less can be achieved.
As a near-field light exposure apparatus, for example, an apparatus may be mentioned in which a mask having an aperture having a dimension of 100 nm or less is placed at a distance of 100 nm or less from a workpiece and in which the workpiece is then exposed with near-field light effused from the aperture. In addition to the apparatus described above, in Japanese Laid-Open Patent Application Nos. 11-145051 and 11-184094, an apparatus has been proposed in which a mask elastically deformable in the direction normal to the surface thereof is closely brought into contact with a resist, and in which, by using near-field light effused from a fine aperture pattern having a dimension of 100 nm or less formed in the mask surface, exposure exceeding the limitation of the light wavelength is performed selectively for the workpiece. According to the publications described above, as means for contacting the mask closely with the resist, the structure is formed in which the difference in pressure is generated between two positions, that is, the front surface side of the mask and the rear surface side thereof, so that the elastically deformable mask is closely brought into contact with the resist, and subsequently, near-field light exposure is performed. As a result, a fine pattern having a dimension of 100 nm or less can be uniformly and simultaneously transferred to the entire surface of the resist on the substrate.
However, although the exposure apparatus using near-field light, described above, is used, when finer patterns having dimensions of 100 nm, 50 nm, and 30 nm are progressively realized, in the L & S mask pattern which is one of the indexes indicating the resolution, the aspect ratio of a resist pattern obtained after development is decreased. Hence, in view of the problem described above, the exposure apparatus using near-field light admits of improvement.
Referring to FIGS. 4A to 4C, the phenomenon described above will be illustrated. FIG. 4A is a view showing the case in which the width of an aperture in a near-field light exposure mask is larger than or equal to the distance between adjacent apertures.
When light for exposure is radiated from above the mask shown in FIG. 4A, the light thus radiated is transmitted as surface plasmons in the aperture having a width smaller than the wavelength. At this stage, the intensity of near-field light exhibits the strongest peaks at two side edge positions of the aperture at the shading film surface side. The intensity of the near-field light is isotropically decreased along the direction away from each edge portion, and at a position at a distance of approximately 100 nm therefrom, the intensity is decreased substantially to zero. This intensity distribution is shown by isointensity lines 110 of the near-field light in FIG. 4A. Since the near-field light generated from one aperture overlaps that from another aperture adjacent thereto, the intensity of the near-field light under the shading film is increased. Accordingly, the difference in intensity of the near-field light between under the aperture and under the shading film is decreased, and as a result, the structure formed in the resist has a smaller aspect ratio. In this case, the resist does not have enough durability as an etching mask, and as a result, a workpiece present thereunder cannot be processed.